The invention relates to a ballast for feeding a fluorescent lamp, comprising supply means to supply a stabilised power to the lamp, dimming means for adjusting the power, a voltmeter which can measure the voltage across the lamp, voltage comparator means which can compare the measured voltage with a threshold, and supply interruption means which can interrupt the power supply to the lamp if the measured voltage exceeds the threshold during a defined delay period.
Such a ballast is described in American patent U.S. Pat. No. 5,043,635. The life of fluorescent lamps, such as TL-lamps, is determined in most cases by wear of the lamp electrodes. Specifically, the emitter powder applied to the electrodes during lamp manufacture will eventually sputter from the electrodes and evaporate. Consequently, the properties of the lamp deteriorate and the light output is less than optimal. Furthermore, the voltage across the lamp increases greatly due to the increased emission potential of the electrodes. This may lead to hazardous situations such as an unacceptable temperature rise of the electrodes, or even melting of the electrodes or immediately adjacent parts. A known method to prevent this situation is to continuously measure the voltage across the lamp and if the voltage exceeds a defined threshold for some time it is assumed that the lamp has reached the end of its life, after which the power supply to the lamp is automatically interrupted. The reason that the measured voltage should exceed the threshold for some time before the lamp is switched off is that the measured signal contains transients, specifically when measurements are made using an AD converter, during which it is desirable for the lamp to continue operating.
Such a method generally provides satisfactory results. However, if the lamp is provided with dimming means to adjust the power flowing through the lamp, a hazardous situation may still arise. This is because the normal operating voltage across the electrodes depends on the power setting and shows a substantially inversely proportional relationship. Thus, the voltage is high at a low power, and falls as the power increases. Thus, for fluorescent lamps provided with dimming means said threshold should be set above the maximum voltage which can occur during normal operation at a low power. Consequently, when the lamp is operated at full power, the threshold will be far above the normal operating voltage. It will therefore take longer for the voltage to exceed the threshold due to electrode wear, as a result of which the lamp may still be overheated.
This invention aims to provide a low-cost, effective, user-friendly and/or safe ballast which is switched off in time when the electrodes are worn out, even if the ballast is provided with dimming means.
To this end, the ballast also includes adjusting means capable of adjusting the magnitude of the threshold in dependence on the power supplied to the lamp, preferably in dependence on the power set by the dimming means or the actually measured power. For example, the threshold could be a fixed percentage above the normal operating voltage for any set power. Consequently, the difference between the normal operating voltage and the threshold will be acceptably low at any power, so that the lamp is switched off in time and hazardous situations are avoided.
As the voltage across the electrodes is furthermore dependent on the temperature of the electrodes in particular, preferably the adjusting means shall also be connected to a temperature sensor which preferably measures the ambient temperature or the temperature of a lamp component, in which case the adjusting means also adjust the threshold in dependence on the measured temperature.
Furthermore, the voltage across the electrodes depends on the type of lamp, and preferably the adjusting means shall therefore adjust the threshold in dependence on the type of fluorescent lamp powered by the ballast. Preferably, the ballast and/or the luninaire in which the lamp is fitted shall therefore contain detecting means to determine the type of lamp.
Preferably, the adjusting means shall calculate the threshold as a function of the set or measured power or the adjusting means shall include storage means in which combinations of powers and the associated thresholds are stored, for example in the form of a table.
The present invention can be implemented efficiently through the use of a microprocessor in the ballast. This makes it straightforward to also determine the delay time in dependence on the power, or the type of fluorescent lamp powered by the ballast, so that in every case the optimum lamp behaviour is obtained at the end of its life.
The present invention also relates to a method of feeding a fluorescent lamp in which the power set by the dimming means is supplied to the lamp, in which the voltage across the lamp is measured, the measured voltage is compared with a threshold, and the power supply to the lamp is interrupted if, during a defined delay period, the measured voltage is higher than the threshold, with the magnitude of the threshold being adjusted in dependence on the power.